The degree of invertebrate faunal similarity across the Pacific Ocean basin generally increased during the Jurassic, Cretaceous, and Cenozoic as the Atlantic widened and the Pacific narrowed. The paleontologic data source is the Treatise on Invertebrate Paleontology, which includes mostly marine genera. The faunal similarities are expressed by the Simpson coefficient, C/N₁, C being the number of taxa occurring in both the American area and the Indopacific-Asian area, and N₁ being the total number of taxa occurring in the area having the smaller number. Genera which occur in Europe and Africa in addition to the two trans-Pacific areas were eliminated to reduce the effect of trans-Atlantic migration, the easier route.

Simpson coefficient values for post-Triassic time intervals when correlated with ocean basin widths estimated from paleogeographic maps, yield a correlation coefficient of 0.956 for the Atlantic and -0.942 for the Pacific. Atlantic widths were used in addition to Pacific widths because paleogeographic control is better in the Atlantic.

The data indicate that the closing Pacific basin had a strong effect on trans-oceanic dispersal. Anomalously high similarity values in the Cretaceous may have been caused by accretionary tectonics around the Pacific margin. The results support the orthodox plate tectonic model rather than expanding earth hypotheses.

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